The present invention generally relates to a method of providing random access for video data in which a P-frame of the video data may be also configured to be random accessible in addition to I-frames.
More specifically, the present invention relates to a method of providing random access for video data in which a P-frame may be also configured to be random accessible based on a random access reference frame which is provided via a separate channel such as header area of video data packets or a metadata file, whereby the random access for video data may be effectively provided with overcoming the conventional video random access technology which depending on the I-frame only.
The video compression technologies, e.g., MPEG, H.264, etc. are utilized in encoding the digital video data (i.e., moving picture) for reducing the data bitrate so as to enable the transmission of the digital video data over network medium.
In an example, the spatial redundancy is removed within a single frame in order to reduce the amount of frame data. This type of encoding is referred to as ‘intraframe (i.e., I-frame)’ encoding, and frame data of the intraframe encoding is referred to as ‘I-frame.’ That is, I-frame is a result of compressive encoding within a single frame data only. Correspondingly, an I-frame may be decoded by itself.
In another example, the temporal redundancy may be removed between frames in order to further reduce the amount of frame data. This type of encoding is referred to as ‘interframe’ encoding, and frame data of the interframe encoding includes P-frame (i.e., Predictive frame) and B-frame (i.e., Bidirectional Predictive frame). The P-frame is a result of compressive encoding of differential parts between the present frame and its previous frame. The B-frame is a result of compressive encoding of differential parts between the present frame and its previous and its next frames.
Correspondingly, a P-frame or B-frame may not be decoded by itself. The P-frame may be decoded with referring to its preceding I-frame as well as a series of its preceding P-frames therefrom. Similarly, the B-frame may be decoded with referring to its preceding I-frame as well as a series of preceding and trailing frames therefrom. By the way, P-frames and B-frames are much smaller than I-frames. In general, a P-frame is about 1/20 of an I-frame in datasize, and a B-frame is about 1/100 of an I-frame in datasize. Therefore, by adopting as many as P-frames and B-frames, the datasize of video data may be minimized.
FIG. 1 is a view illustrating frame format of video data of prior art. The digital broadcasting video generally includes I-frames, B-frames and P-frames. However, the present invention is directed to propose an idea of making a P-frame random accessible. For convenience of explanation only, B-frames are omitted in this specification. Therefore, the specification describes I-frames and P-frames, the present invention may be applied to video data including I-frames, B-frames and P-frames.
Referring to FIG. 1, video data includes a series of frames, in which most of the frames are P-frames 111, 112, . . . with small number of I-frames 110, 120. Because I-frame may be decoded by itself, the I-frames 110, 120 are arranged in positions where random access is intended. For example, in order to make video random accessible in every second, in case of 30 FPS (Frame-Per-Second) video, an I-frame shall be arranged in every 30 frames.
As shown in FIG. 1(a), I-frames 110, 120 are random accessible because the I-frame may be decoded by itself.
However, as shown in FIG. 1(b), P-frames 111, 112, . . . are random inaccessible. For example, in order to decode the P-frame 117, the video decoder shall decode not only its closest preceding I-frame 110 but also a series of its preceding P-frames 111˜116 therefrom. In case of decoding a P-frame, the number of preceding P-frames to decode is variable depending on the location of the P-frame in the frame sequence, which results in ununiform decoding time. That is, uniform response time is not guaranteed for P-frames, that is why the P-frame is regarded to be random inaccessible.
By the way, an I-frame is much bigger than a P-frame in datasize. Because data compression ratio is critical in video data in view of network bandwidth and storage space, most of frames are configured in P-frames and smaller numbers of I-frames are arranged to support the random access in every 0.5 or 1.0 second.
In most of technical fields, the conventional art as described above may be adopted without any problem. Rather, it may be accepted as proper harmony of data compression ratio and random accessibility.
However, in specific fields, e.g., the security control system, the conventional art may result in unsatisfactory effect. The video taken by CCTV cameras has little change between frames in most of the time. As a result, P-frames are so tiny, and accordingly the difference in datasize between a P-frame and an I-frame is remarkable. Therefore, considering the efficiency of storage space of CCTV video, it is preferable to decrease the rate of I-frames in CCTV video.
By the way, due to the nature of security control system, the random accessibility is a crucial requirement. When a staff member urgently requests to check a specific playback point for a security purpose, the system shall provide random access to the playback point or at least just near the playback point. It would be a big problem if the system provides random access only 25 seconds before or 35 seconds after the playback point. Therefore, considering the requirement of random accessibility, it is preferable to increase the rate of I-frames in CCTV video.
Finally, in special fields, e.g., security control system, there are many difficulties in video data processing. Considering storage space for storing the massive video data from great numbers of CCTV cameras, it is preferable to decrease the rate of I-frames with mostly filling the video data with P-frames. Then, however, the random accessibility would be too poor to utilize the CCTV video at the urgent situations. That is, the CCTV video would become useless. On the contrary, if the rate of I-frames is increased in order to improve the random accessibility, the storage space shall be greatly increased too.
Due to the trade-off between random accessibility and space storage, in conventional security control systems, the I-frames are produced in CCTV video with reasonable time interval (e.g., 0.5 or 1 second) so as to make the CCTV video random accessible with the same time interval. In general, the time interval is chosen in view of the random accessibility, which imposes big burden in storage space. The burden in storage space is expected to become bigger, because there are social requirements that much more CCTV cameras must be installed and the resolution of CCTV cameras must be enhanced.